A multicarrier optical communication system based on discrete multi-tone (DMT) modulation is deemed as a major competing technology of a next generation short-range optical communication due to such features as high transmission rate, simple hardware structure, and low power consumption, etc., and interconnection of large-scale computers in a data center is a preferred application scenario of the multicarrier optical communication system based DMT. The DMT technology has been widely used in wireless communication and access networks; however, the scenario of data center has a higher requirement on stability of a transmission performance. In order to guarantee continuous stable transmission performance, it is necessary to monitor signal quality of carriers in a real-time manner during the transmission and perform corresponding adjustment according to a monitoring result. The monitoring of the signal quality is carried out by measuring signal to noise ratios of the subcarriers. Measurement of a signal to noise ratio in the known art may be divided into two types, pilot-based measurement and data-decision-based measurement, and calculating methods of which may be expressed by formulae (1) and (2):
                              SNR          =                                    E              ⁡                              (                                                                                                S                      rx                                                                            2                                )                                                    E              ⁡                              (                                                                                                                        S                        rx                                            -                                              S                        tx                                                                                                  2                                )                                                    ;                            (        1        )                                SNR        =                                            E              ⁡                              (                                                                                                S                      rx                                                                            2                                )                                                    E              ⁡                              (                                                                                                                        S                        rx                                            -                                                                        S                          ^                                                tx                                                                                                  2                                )                                              .                                    (        2        )            
In the two formulae, Srx is a signal received by a receiver. It can be seen from comparison of formulae (1) and (2) that a difference between them is that which of the following two values is based on when calculating the noise, a known transmission signal (Stx) or decision estimation (Ŝtx) on a transmission signal.
The pilot-based measurement of a signal to noise ratio requires a transmitter to insert a signal known to a receiver into transmission data, and the receiver calculates a signal to noise ratio according to formula (1). As Stx is a known signal, the pilot-based measurement of a signal to noise ratio may provide an accurate result. However, as the pilot occupies a bandwidth for data transmission, which has an effect on quality of the data transmission, insertion of a pilot signal should not be so frequent, and a generally-agreed proportion is about 1%-5%. That is, the pilot-based monitoring of signal quality can only provide a 1%-5% of time sampling result, but cannot provide temporarily continuous real-time in-line monitoring.
And on the other hand, the data-decision-based monitoring of signal quality estimates a transmitted signal by directly performing decision on received data information, which may provide continuous in-line monitoring. However, due to errors possibly existed in the data decision, the data-decision-based monitoring of signal quality cannot provide an accurate measurement result, which is more obvious when a modulation format is relatively high or a noise is relatively large.
It should be noted that the above description of the background is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the present disclosure.